Improved latent heat storage device

ABSTRACT

A rapid absorption and extraction latent heat storage device according to the invention comprises a means ( 1 ) for containing at least one phase change material ( 2 ) within a containment vessel ( 3 ), the containment vessel forming a supporting structural exoskeleton. The containment means ( 1 ) is in the form of rods or tubes, which are made out of elastomeric material ( 4 ) containing the PCM ( 2 ). A heat exchange fluid ( 6 ) flows between and along the length of the rods ( 1 ).

The invention relates to the efficient storage, with rapid absorption and extraction, of thermal energy.

In recent years the need to store energy, usually electrical, has increased significantly. With this invention it is now possible to efficiently store thermal energy which can be utilised to generate electricity, cooling, heating or for what ever purpose at a later date.

There have been various inventions such as WO8900670 (Whitman) which show various ways of storing thermal energy utilising Phase Change Materials (PCM) but they all have fundamental problems. With the wax varieties of PCM the main problems centre on its poor thermal conductivity and relatively large coefficient of thermal expansion.

For Eutectic and Salt Hydrate type PCM's the thermal conductivity is significantly better but corrosion can be a major problem. All of the current PCMs have expansion and contraction issues which make the containment of these materials extremely difficult, especially when trying to get thermal energy in and out of the PCMs quickly and efficiently, using the smallest volume possible. Existing methods have employed strong thick materials; 100 mm diameter balls made out of 3.0 mm thick stainless steel and thick plastic mouldings, most of which have to leave air gaps to allow for some expansion and minimise the stresses on the containment materials.

There have been successful demonstrations using Stainless Steel balls and tubing containing PCM. However in order to get the thermal energy into and out of these stores they have had to be very large, or have complex fluid distribution and control systems, so that the slow absorption and extraction rates can be accommodated. Some attempts at including conductive rings and elaborate finning on complex heat exchangers have been attempted, especially with the wax variety of PCM. This invention significantly improves the heat transfer rates even if the PCM is not very thermally conductive.

U.S. Pat. No. 6,889,751 (Lukas) shows a polygon shaped structure as this is well understood as being the best way of getting the maximum volume of material into a known space when using tubes. However the invention does not take account of the properties of the PCMs being used. Using small bore pipes to pass the heat transfer fluid through and surrounding these tubes with PCM is not the most efficient use of the space, even if the tubes are finned. Alternatively by putting the PCM into the tubes may be an improvement but the ends of the tubes have to be sealed and allow for thermal expansion etc.

It is an object of the present invention to increase the density of storage of thermal energy in a given volume and to facilitate the speed of absorption and extraction of that energy.

The invention provides a latent heat storage device in accordance with claim 1 of the appended claims. The latent heat storage device contains at least one phase change material (PCM) contained within containment means and comprises a containment vessel forming a supporting structural exoskeleton to provide support for the containment means. The containment means comprises very thin elastomeric material and is of a wall thickness that is much thinner than has previously been contemplated in the art. The thinness of the walls enables very efficient heat transfer to and from the PCM.

The thin elastomeric material can be formed into any shape of thin section and providing a very large surface area to volume ratio, for example having many sided or circular chambers, and provided the distance through any section of PCM is small enough to effect rapid melting and freezing of the PCM.

In a particularly advantageous form the containment means comprises thin elastomeric material formed as a continuous tube, filled with PCM and sealed at both ends, then folded along its length to occupy the maximum amount of space that is available within the containment vessel. This arrangement allows the amount of sealing required to be minimised and provides a very efficient means of maximising the amount of space used within the vessel by the containment means.

An alternative advantageous form comprises an array of tubes joined by small web sections. This type of array can be formed by moulding or extrusion or can even be made using 3D printing technology techniques. The tubes can be sealed at each end and enables a maximum amount of space to be occupied within the vessel by the tubes.

The elastomeric material is selected to have as thin a wall thickness as possible to structurally retain the PCM and at the same time to provide the minimum effect on the transfer of heat to and from the PCM. This enables the device of the invention to have a very rapid response time and to absorb and provide energy very quickly. Additionally it is effective even for very small temperature differences.

There are small gaps between the PCM filled elastomeric material to allow heat exchange fluid to pass between said material to facilitate the absorption and extraction of thermal energy. The gaps form small cross-section flow-channels enabling efficient flow of heat exchange fluid through the vessel.

A number of different arrangements of the containment means and the PCMs within the vessel are possible. Advantageously the device may have a multiplicity of different PCM, with different properties, within the one vessel. The device may also have a multiplicity of different compartments within the one vessel, either with PCMs that are the same or that are different. The compartments may be formed using insulated or non-insulated panels. Advantageously the flow of heat exchange fluid is controlled through the vessel and it may be directed to the different compartments in turn. The flow of fluid can be directed to different parts of the device to accommodate different requirements at different times.

In an alternative arrangement, the device may have a multiplicity of vessels within the one device.

In a preferred embodiment of the invention, the latent heat storage device vessel has a sealed lid. If the device is sealed, then gas or fluid can be injected into the sealed vessel to effect a different atmosphere or environment such as a reduced oxygen atmosphere for the benefit of any heat transfer fluid or other material's needs for a reduction in oxidation. Advantageously the inert gas may be nitrogen or carbon dioxide.

In another preferred embodiment of the invention, the device has a means whereby heat exchange fluid is supplied to the vessel and removed from the vessel, so as to be a closed circuit such that whatever fluid is supplied is also removed at the same time to avoid overfilling or emptying of the vessel.

In a particularly advantageous embodiment of the invention, the elastomeric contained PCM is allowed to expand and contract according to its nature. In particular, the expansion and contraction can take place initially out of and back into the vessel, preferably the top of the vessel. Following the expansion of the PCM and hence the containment means into the top of the vessel, the containment means may subsequently expand into the heat exchange channels within which the heat exchange fluid flows. This arrangement has the particular advantage in that it can be used to provide an automatic limiting of the flow of fluid as the expansion acts to progressively restrict the flow of heat exchange fluid between the elastomeric tubes. This provides a particularly useful safety mechanism to prevent overheating of the PCM, elastomeric and/or other materials used.

The vessel is arranged to provide the exoskeleton structural integrity for the elastomeric PCM, once the PCM has melted. If the device has internal compartments or dividers, these can also be used to provide the structural integrity for the elastomeric PCM, once the PCM has melted. This enables a far thinner wall thickness of the containment means to be used than has previously been achievable.

In many arrangements of the invention it will be advantageous for the vessel to be surrounded by insulation. This may be any suitable insulating material including vacuum insulation. Additionally the device may be surrounded by a secondary insulated tank filled with water or any other suitable fluid, such that any thermal energy that escapes from the inner vessel or vessels will be absorbed and there will be minimal loss to the surrounding atmosphere.

The thermal conductivity of certain PCMs may be improved by adding very small quantities of very fine powders of suitable conductive material to the PCM. If the particle sizes are small enough then they will remain suspended within the main body of the PCM. They will also have a tendency to get continually redistributed by any convection currents induced by the melting of the PCM.

The invention further provides a latent heat storage device comprising a containment vessel, at least one phase change material (PCM) and at least one PCM containment means wherein at least one very fine nano-particle conductive powder is added to the PCM to improve the transfer of thermal energy. The addition of very fine nano-particle conductive powders can significantly improve the performance of poorly conducting PCMs. Suitable examples include, but are not limited to, carbon and aluminium. The concentrations can vary depending upon the materials used but typically can be anything from 0.5% to 2%; larger concentrations may well reduce the amount of PCM volume and influence the overall performance. It is now possible to make up new composite materials utilising the properties of the different components to maximise the thermal capacity and heat transfer rates. Although it is possible to improve the situation the current technical specifications are exacting as there is a tendency for these materials to settle or separate out with time and the present invention enables these problems to be overcome.

In another alternative form of the invention, the vessel may be surrounded by secondary layers of different PCM filled elastomeric material. Advantageously, but not essentially, the PCM of the secondary layer may have a lower phase change temperature than the PCM of the vessel.

If the latent heat storage device of the invention is used with a solar heating device or other device, it can be arranged so that the heat exchange fluid feed-back temperature is lower than it would otherwise be; so as to maximise the efficiency of the solar heating or other device.

The invention will now be described by way of example only with reference to the accompanying drawings, of which:

FIG. 1 shows a schematic cross-section of a latent heat storage device according to the invention.

FIG. 2 shows a more detailed view of a corner of the device shown in FIG. 1.

FIG. 3 shows an example of a containment means in the form of a continuous round tube folded into six rods.

FIG. 4 shows an embodiment of the invention in form of a “tank within a tank”.

FIG. 5 shows an embodiment of the invention, wherein separators can be used to direct the flow of heat exchange fluid through different compartments of PCM.

FIG. 6 shows an alternative shaped containment means, with detail shown in FIG. 6 a.

FIG. 7 shows a further alternative shaped containment means, with detail shown in FIG. 7 a.

FIG. 8 shows a possible packing arrangement of containment means of the type shown in FIG. 3.

The various embodiments of the invention overcome most or all of the hereinbefore mentioned problems.

As shown in FIG. 1, a rapid absorption and extraction latent heat storage device according to the invention comprises a means 1 for storing a phase change material 2 arranged within a containment vessel 3 (for which insulation is not shown in this example). The containment means 1 is in the form of rods or tubes, which are made out of elastomeric material 4 containing the PCM 2. A heat exchange fluid 6 flows along the length of the rods 1. The PCM 2 may advantageously have a fine nano-particle conductive powder such as carbon or aluminium added to improve its conductivity.

The containment vessel 3, is shown as hexagonal but could be any shape that maximises the storage capacity for round rod-like multiple components 1 but the rods can be any shape provided they offer a thin enough section, for conduction, to enable heat transfer through the whole section in an acceptable time frame. In order to accommodate the expansion and contraction the outer skin of the rods is made from an elastic material 4 which utilises the close proximity of all the adjacent rods to support them when the PCM 2 is in the liquid phase. The rods 1 have a small diameter and are long. In a particular example of the invention, the tubes have an external diameter of 10.5 mm and an internal diameter of 10.0 mm, and as such have a wall thickness of only 0.25 mm. In contrast, WO 95/16175, for example, describes tubes of HDPE having an outer diameter of 38 mm and an internal diameter of 32 mm and thus having a wall thickness of 3.0 mm. This thick a wall will limit the heat transfer that is possible between the heat exchange fluid and the PCM within the tube. Entry/exit pipes 13 a allow the heat exchange fluid to be fed into and removed from the vessel 3.

FIG. 2 shows the detail of a corner of a containment vessel 3. The heat exchange fluid 6 passes through the spaces in between the rods 1. In the configuration shown in FIG. 1, it is possible to get about 90% of the volume filled with PCM 2. The tubular elastomeric material 4 used for the containment of the PCM 2 must be thin enough to not take up too much volume and also to conduct thermal energy efficiently. Initial prototypes have shown that many kilo watts of thermal energy could be stored and released in only a few minutes.

FIG. 3 shows a preferred example of a containment means of the invention. The containment means comprises a continuous tube 7, of preferably circular cross-section, folded into six rods 1. Where the folds take place at the top and bottom of the rods, while the PCM 2 is molten, this area can be shaped to provide the round rod like shape.

In one particular embodiment, the containment means 1 was made from one length of tubing nominally 4500 mm long. In this example there are only two seals 8 needed for each batch of six rods. The containment means 1 can be any desired length but handling and the strength of the tubing will create a practical limit.

In one embodiment of the invention, a small single stack of rods 1 made with about 30 kg of salt hydrate PCM 2, having a phase change temperature of 58° C., flow rates of heat exchanger fluid 6 of over 2 litres/min at temperatures over 100° C., absorbed all the thermal energy contained within the heat exchange fluid 6 and the exhaust temperature did not start to rise above room temperature until about 75% of the PCM 2 had already reached melting point.

FIG. 4 shows an embodiment of the invention in form of a “tank within a tank”, where two different types of PCM 2 a, 2 b are housed within the same tank and separated by separators 9, which in this example are an internal block and an external block where the separators 9 are suitable internal insulation; typically capable of withstanding higher temperatures. Similarly external insulation 10 is also shown. Insulation can be made from any suitable materials including Vacuum Super Insulation.

FIG. 5 shows a different arrangement, where separators 11 are arranged within the containment vessel 3. The separators 11 can be used to direct the flow of heat exchange fluid 6 through the different compartments of the vessel 3 which can utilise different types of PCM 2 having different melting temperatures should this be required. Similarly, as with FIG. 4, the separators could be made out of any suitable insulation materials.

The tubes 2 in this arrangement can advantageously be formed as an extruded array of tubes connected by thin webs between each adjacent tube. Each section in the containment vessel 3 has an array 50 fitted within the section. The arrays 50 can be formed by moulding or extrusion or other suitable means. A single compartment vessel as shown in FIG. 1 may have a single array 50 or a combination of two or more arrays to fill the vessel. Similarly each section of the vessel shown in FIG. 5 may have one or more arrays to fill the sections of the vessel.

Alternative shaped rods of wide flat configuration of a sheet of tubes connected by very thin webs or circular tubes in the form of concentric rings linked by thin web sections, as shown in FIGS. 6 and 7 can also be used. The critical configuration is that the section of PCM 2 between the heat exchange fluid exposed surfaces must be no thicker than that to ensure rapid melting and solidifying. Because of the thin sections of material very large surface areas for heat transfer are possible and it was found to be ideal for the eutectic and salt hydrate materials used in the prototypes constructed. It is still effective with plain waxes but the sections have to be thinner unless fine nano-particle conductive materials are added. In the non-round rod configurations, the channels for the flow of heat exchange fluid 6 will need small separators 12 to ensure the channels remain open until over temperature situations arise. Similarly, if alternative shaped rods are used, the periphery of the whole block or assembly of materials will need the external surface to be shaped so as to allow heat exchange fluid to flow between the containment vessel 3 and the outer skin 4 of the elastomeric contained PCM 2. Provision for the overall movement heat exchange fluid 6 can be made by the provision of pipes or channels 13, an example is shown in FIG. 1.

FIG. 6 shows a rod having a rectangular cross-section. In this case, the section depicted is effectively where the round rods depicted in FIG. 1 are joined together. With this kind of design it is possible to actually get more PCM into any given volume, but this would be by the diminution of the heat exchange fluid. Consequently higher velocities of flow will be encountered but it is anticipated that over 95% of the volume could be effectively occupied with PCM.

FIG. 7 gives another example of different shaped rods. In this case the vessel 3 is round and the elastomeric rods 14 are similarly shaped and are arranged as concentrical rings linked by small web sections 12 with heat-exchange fluid 6 flowing between the layers. With any configuration of this type it will normally be necessary to ensure a non-uniform perimeter of the assembled rods, so as to allow heat exchange fluid to pass between the vessel and the perimeter. In the embodiment shown, this is in the form of a “corrugated” type outer surface 15, which can be seen in more detail in FIG. 7 a. The flow path could alternatively be accommodated by varying the shape of the vessel but the essential feature is that the rods are still supported and contained by the vessel as it acts as an exoskeleton for the main structure.

It is an additional feature of the invention that it tends to be self regulating as the currently used PCM expands. Initially it will expand mainly vertically, until it reaches the top of the vessel, then it will tend to expand into the spaces between the elastomeric containment means and it then starts to restrict the flow of fluid through the stack of rods. Should the temperature of the heat exchange fluid get too high for the materials contained, then by design, the flow can be cut off completely. In a similar manner if alternative shaped rods (not round) are used then some supporting ribs 12 will be needed to prevent the heat exchange channels from collapsing during normal operation. In principle this invention will facilitate the maximum speed of input and extraction of thermal energy what ever materials are encased within the elastomeric material. It is therefore possible, by balancing the pressures across the elastomeric material, to have open-ended tubing or channels and to use the elastomeric material as a conventional heat exchanger.

FIG. 8 shows a packing arrangement of a plurality of tubes 7 of the type shown in FIG. 3, each folded into six rods 1 and close packed within a hexagonal containment vessel 3. A single entry/exit pipe 13 b allows the heat exchange fluid 6 to flow in and out of the vessel 3, as required.

Initial prototypes have been constructed using PCMs that have state changes at 58° C. and 89° C. but the basic principle will work with any temperature. Even heat transfer fluid temperature differences of only 4° C., above or below the phase change temperature, will affect efficient heat transfer in only a few minutes.

In this example the elastomeric material 4, containing the PCMs 2, is provided in the form of an extruded tube but the elastic film surrounding the PCMs can be sprayed on to the PCM material or the PCM material can be dipped into a solution. It is also possible to construct the elastomeric material using the 3D printing technologies so that the complete structure can be made as one unit, or as an assembly of smaller units. 

1. A latent heat storage device comprising: at least one containment vessel forming a supporting structural exoskeleton; at least one phase change material (PCM); and at least one PCM containment means comprising an elastomeric material wall, the elastomeric material being selected to have as thin a wall thickness as possible to structurally retain the PCM and at the same time to provide the minimum effect on the transfer of heat to and from the PCM by way of improved thermal conductivity, the PCM containment means containing the PCM and wherein the PCM can expand and contract as it absorbs or gives up heat in use; wherein the at least one PCM containment means is arranged within the containment vessel such that there are channels between at least parts of the PCM containment means to allow heat exchange fluid to pass between said containment means to facilitate the absorption and extraction of thermal energy from the PCM within the containment means and wherein the containment vessel provides a supporting structural exoskeleton to the elastomeric walls of the containment means, when the PCM is in a melted state in use.
 2. A latent heat storage device according to claim 1 wherein the containment vessel comprises inlet means and outlet means to allow flow of heat exchange fluid through the containment vessel.
 3. A latent heat storage device according to claim 1, wherein the expansion and contraction of the PCM causes the elastomeric material of the containment means to expand and contract firstly in and out of the top of the vessel and subsequently in the heat exchange channels within which the heat exchange fluid flows.
 4. A latent heat storage device according to claim 3 wherein the expansion acts to progressively restrict the flow of heat exchange fluid to providing a safety mechanism to prevent overheating of the PCM, elastomeric and or other materials used.
 5. A latent heat storage device according to claim 1 wherein the thin elastomeric material is formed into chambers of thin section having a circular, multi-sided or other shaped cross-section, filled with PCM.
 6. A latent heat storage device according to claim 1 wherein the containment means comprises at least one continuous tube of elastomeric material, filled with PCM and sealed at both ends, the containment means being folded along its length to form a series of connected portions and being positioned within the containment vessel.
 7. A latent heat storage device according to claim 1 wherein the containment means comprises thin elastomeric material formed as an array of tubes connected by web sections, the tubes being filled with PCM and sealed at both ends.
 8. A latent heat storage device according to claim 7 wherein the tubes are of circular, multi-sided or other shaped cross-section.
 9. A latent heat storage device according to claim 7 wherein the array of tubes is an extruded array.
 10. A latent heat storage device according to claim 1 wherein the containment means comprises thin elastomeric material formed as an array of concentric rings linked by web sections to enable heat exchange fluid to flow between them, the rings being filled with PCM and sealed at both ends and the flow channels for the heat exchange fluid being between each concentric ring.
 11. A latent heat storage device according to claim 10 wherein the outermost ring adjacent the wall of the containment vessel comprises a corrugated arrangement of channels of PCM linked by web sections to enable heat exchange fluid to flow between the outer surface of the outermost ring and the wall of the containment vessel.
 12. A latent heat storage device according to claim 1 wherein very small quantities of very fine powders of suitable conductive material are added to the PCM to improve the thermal conductivity.
 13. A latent heat storage device according to claim 12 wherein the powder comprises fine nano-particles of conductive powder.
 14. A latent heat storage device according to claim 13 wherein the powder comprises fine nano-particles of carbon or aluminum.
 15. A latent heat storage device according to claim 1 wherein the device has a multiplicity of different PCM, with different properties, within different containment means within at least one containment vessel.
 16. A latent heat storage device according to claim 1 wherein the device has a multiplicity of different compartments within at least one vessel.
 17. A latent heat storage device according to claim 16 comprising means to direct a flow of heat exchange fluid to the different compartments.
 18. A latent heat storage device according to claim 16 comprising means to control the flow of heat exchange fluid such that the flow of fluid can be directed to different parts of the store to accommodate different requirements at different times.
 19. A latent heat storage device according to claim 16 wherein the compartments provide structural integrity for the elastomeric material of the containment means, once the PCM has melted in use.
 20. A latent heat storage device according to claim 1 wherein the device has a multiplicity of vessels within the one device.
 21. A latent heat storage device according to claim 1 wherein the vessel has a sealed lid.
 22. A latent heat storage device according to claim 21 wherein the device has a gas or fluid within the sealed vessel to effect a different atmosphere or environment.
 23. A latent heat storage device according to claim 21 wherein the gas is nitrogen or carbon dioxide.
 24. A latent heat storage device according to claim 1 wherein the device has a means whereby heat exchange fluid is supplied to the vessel and removed from the vessel, so as to be a closed circuit such that whatever fluid is supplied is also removed at the same time to avoid overfilling or emptying of the vessel.
 25. A latent heat storage device according to claim 1 wherein the vessel is surrounded by a secondary additional layer such as PCM, water or other suitable material, such that any thermal energy that escapes from the inner vessel or vessels will be absorbed and there will be minimal loss to the surrounding atmosphere.
 26. A latent heat storage device according to claim 25 wherein the secondary additional layer comprises a PCM having a lower phase change temperature than the PCM of the vessel. 